The present invention is an automated method for assisting lumber grading. It involves determination of the pith location of the log from which the lumber was cut relative to the lumber surfaces. Having this knowledge, the orientation of knots within the lumber can be ascertained and the cross sectional area of the knots more readily estimated. In its most preferred form, the method assigns a tentative grade to the lumber.
Several criteria are used in grading lumber products. Lumber to be used for furniture, mill work or similar products is graded on the basis of appearance with relatively little consideration being given to defects which could affect strength. On the other hand, appearance is given essentially no weight in grading lumber products intended for structural use. The lumber grades are assigned on the basis of the expected strength in bending, compared with that of similar defect-free products. Most grading rules in the United States are based on ASTM Test Method D 245-81. This publication notes that lumber grades "designate near-minimum strength and near-average stiffness properties on which to base structural design."
Virtually all structural lumber today is visually graded. This is done by examining all four faces and the ends of the piece and by noting the location as well as the size and nature of knots and other defects over the entire length. As noted earlier, lumber grades are based on the strength ratio of a structural member compared to the strength it would be expected to have if no weakening characteristics were present. The various factors that affect strength are knots, grain deviations such as diving grain, shake, and checks or splits. Certain other factors such as various types of warpage, density, and the presence or absence of surface defects such as planar skip or wane also enter into the grading process.
Most structural lumber in the United States is graded with the assumption that it will be used as a joist; i.e., with bending loads applied to one of the narrow faces of the lumber. Other grading rules apply to lumber designated for use as planks; i.e., with the bending load applied to one of the wide faces. Some lumber is graded for use in compression, wall studs being an example.
While all of the defects noted previously are important, the presence of knots is generally the principal reason for downgrading lumber. To again quote from ASTM D-245 "Strength ratios associated with knots and bending members have been derived as the ratio of moment-carrying capacity of a member with cross section reduced by the largest knot to the moment-carrying capacity of the member without defect. This gives the anticipated reduction in bending strength due to the knot. For simplicity, all knots on the wide face are treated as being either knots along the edge of the piece (edge knots) or knots along the centerline of the piece (centerline knots)." Thus, in grading a piece of lumber the grader must visually and very rapidly note the number, size and position of knots, estimate their cross sectional area, and mentally determine how much they will reduce the strength of the piece. In addition, he must also factor in all of the other flaws which affect grade. He must do all of this in a period of only a few seconds, mark the grade on the lumber and then move on to the next piece.
Grading is a job that requires very high skill. Graders are routinely examined by a head grader from one of the industry supported grading bureaus and are typically required to assign correct grade at least 95% of the time.
In the past, many attempts have been made at machine grading lumber. Probably the simplest of all methods is to bend the lumber as a plank in flexure and determine modulus of elasticity. Pieces that fall below a given threshold level are diverted to the lower grades. Other more sophisticated methods involve scanners which determine the presence and location of knots and/or other defects and assign a grade on this basis. One group of scanners detects knots on the basis of their color in comparison to the adjacent wood. Stated otherwise, scanning is based on a gray scale with the darker colors representing knots and the lighter colors representing knot-free wood. One early method is disclosed in U.S. Pat. No. 3,120,861 to Finlay et al. Another group of automated graders also involves human intervention with the scanning system. One such method is described in U.S. Pat. No. 3,329,181 to Buss et al. Here a human inspector marks the flaws prior to scanning and may input information to the computer describing the nature of the flaw; e.g., that it might be acceptable in some lumber grades and unacceptable in others. Barr et al in U.S. Pat. Nos. 3,931,501 and 3,942,021 include in their background a good description of prior art scanners which are associated with a computer for decision making. The Barr scanners also require human input for designating the defect type which must be marked on the board prior to scanning. In this system the scanner unit moves along the workpiece and uses an ultraviolet light and photocell array to detect defects, including those marked by the inspector. U.S. Pat. No. 4,163,321 to Cunningham relates closely to the system of Barr et al.
Another scanning system for grading lumber which requires human intervention is described in U.S. Pat. Nos. 4,149,089 and 4,221,974 to Mueller et al. The Mueller system uses a swept laser beam to examine both faces of a board. A human inspector serially inputs the class of flaw to the computer as it passes the scanning station. The human inspector may enhance a flaw for detection by the scanner or may suppress certain characteristics which the scanner might detect as an objectionable flaw. The computer then makes optimum cutting decisions by comparing the board characteristics with an order file. U.S. Pat. Nos. 4,149,089 and 4,207,472 to Idelsohn and U.S. Pat. No. 4,286,880 to Young show systems which are similar in many respects to those described by Mueller et al.
The prior art systems noted to date are predominantly interested in detecting defects which affect the visual appearance of wood and little emphasis is placed on defects which affect structural properties. In these systems the wood is primarly intended for use in furniture manufacture and may be very high value hardwoods so that optimum cutting decisions for remanufacture can be of great economic importance.
A somewhat different type of scanner for wood is disclosed in PCT Application No. WO85/00657. Here the scanners examine at least one end and one longitudinal surface. One set of sensors have coarse resolution while another set has fine resolution. The fine resolution sensors are capable of movement so that they can examine specific areas of the lumber noted by the other sensors. This also is a gray scale scanner. However, the inventors note that other defects besides knots, such as pith streaks, can be recognized.
A different scanning system is disclosed in U.S. Pat. No. 3,976,384 to Matthews et al. Here light is injected into the surface of lumber at one point and the emerging light is detected at a spaced apart location. The system can detect defects such as knots, blue stain, and certain types of rot. The inventors have noted that in clear (defect free) wood, light traveling across the grain is attenuated by a factor almost 50 times greater than light traveling along the grain. The method senses surface fiber direction changes on the plane of the face being measured. However, it has been found to be unsuitable for use on the woods of deciduous, or so-called "hardwood," species.
Another type of system using microwave scanning is found in U.S. Pat. Nos. 4,123,702 to Kinanen et al and 4,607,212 to Jakkula. Kinanen uses high frequency microwave energy to detect knots based on their differing dielectric constant from sound wood. Jakkula similarly uses microwave radiation to detect knots by noting a mode or polarization shift from TEM to TM which is detected by a receiver. This inventor notes that knots can be detected in a wane edge but does not comment further on the this observation.
Another group of Finnish inventors, Hirvonen et al, in U.S. Pat. No. 4,482,250 scan a wood surface with a polarized beam of light having the plane of polarization either parallel to or perpendicular to the longitudinal axis of the piece of wood. The light reflected from a piece of sound wood is only partially depolarized while the light reflected from a knot is almost totally depolarized.
Davis et al, in U.S. Defensive Publication No. T932,008 disclose an optical system for measuring surface fiber direction in a moving web structure. These authors note that when collimated light is projected on a fibrous web, the light will be reflected with greatest intensity perpendicular to the fiber axes. Reflectance values measured along and across the machine direction of the web can be used to determine the average orientation of the fiber on the surface in two dimensions.
An advanced scanner which can determine fiber angle in three dimensions from a surface examination is described in Matthews et al, U.S. Pat. No. 4,606,645. This is commonly assigned with the present invention and is incorporated herein by reference. This Matthews invention measures fiber angle in a fibrous solid material relative to three mutually orthogonal reference axes. It is particularly well suited from measuring diving grain and grain surface angle in wood. This scanner employs a laser beam which may be mechani-optically swept transversely across the surface of a longitudinally moving piece of lumber. Reflections from the surface are measured by an array of photo detectors lying preferably in a common plane around the illumination source. The method can very readily pick up aberrations in grain angle such as those caused by knots below the surface or by visible knots. Additionally, it can readily detect other defects such as planar skip, wane, and splits.
Other scanners have been concerned with detection of wane on lumber. Wane is here defined as an original unsawn surface of the log which truncates one or more edges of the piece of lumber. U.S. Pat. No. 3,983,403 to Dahlstrom et al describes a system for detecting and measuring wane. Another system which can recognize wane is described in U.S. Pat. No. 4,188,544 to Chasson.
While all of the above-noted inventions are useful in some way for scanning wood surfaces and providing quality information, none of them have been developed into apparatus which can do structural grading at the very high speeds required in a modern lumber mill. In fact, those which require human intervention are in many cases no faster or more accurate than visual grading. The present invention has overcome the problems inherent in the prior art methods and is capable of accurately grading structural lumber at the high speed found in modern sawmills. In advanced versions of the invention, no further human intervention is necessary, while in simpler versions a tentative grade is proposed which may be approved or modified by a downstream human inspector. In either circumstance the amount of human labor and decision making involved is greatly reduced over that required for manual grading.